class ActiveSupport::Concurrency::ShareLock
A share/exclusive lock, otherwise known as a read/write lock.
Public Class Methods
# File lib/active_support/concurrency/share_lock.rb, line 49 def initialize super() @cv = new_cond @sharing = Hash.new(0) @waiting = {} @sleeping = {} @exclusive_thread = nil @exclusive_depth = 0 end
Public Instance Methods
Execute the supplied block while holding the Exclusive lock. If
no_wait
is set and the lock is not immediately available,
returns nil
without yielding. Otherwise, returns the result of
the block.
See start_exclusive
for other options.
# File lib/active_support/concurrency/share_lock.rb, line 147 def exclusive(purpose: nil, compatible: [], after_compatible: [], no_wait: false) if start_exclusive(purpose: purpose, compatible: compatible, no_wait: no_wait) begin yield ensure stop_exclusive(compatible: after_compatible) end end end
Execute the supplied block while holding the Share lock.
# File lib/active_support/concurrency/share_lock.rb, line 158 def sharing start_sharing begin yield ensure stop_sharing end end
Returns false if no_wait
is set and the lock is not
immediately available. Otherwise, returns true after the lock has been
acquired.
purpose
and compatible
work together; while this
thread is waiting for the exclusive lock, it will yield its share (if any)
to any other attempt whose purpose
appears in this
attempt's compatible
list. This allows a “loose” upgrade,
which, being less strict, prevents some classes of deadlocks.
For many resources, loose upgrades are sufficient: if a thread is awaiting
a lock, it is not running any other code. With purpose
matching, it is possible to yield only to other threads whose activity will
not interfere.
# File lib/active_support/concurrency/share_lock.rb, line 75 def start_exclusive(purpose: nil, compatible: [], no_wait: false) synchronize do unless @exclusive_thread == Thread.current if busy_for_exclusive?(purpose) return false if no_wait yield_shares(purpose: purpose, compatible: compatible, block_share: true) do wait_for(:start_exclusive) { busy_for_exclusive?(purpose) } end end @exclusive_thread = Thread.current end @exclusive_depth += 1 true end end
# File lib/active_support/concurrency/share_lock.rb, line 113 def start_sharing synchronize do if @sharing[Thread.current] > 0 || @exclusive_thread == Thread.current # We already hold a lock; nothing to wait for elsif @waiting[Thread.current] # We're nested inside a +yield_shares+ call: we'll resume as # soon as there isn't an exclusive lock in our way wait_for(:start_sharing) { @exclusive_thread } else # This is an initial / outermost share call: any outstanding # requests for an exclusive lock get to go first wait_for(:start_sharing) { busy_for_sharing?(false) } end @sharing[Thread.current] += 1 end end
Relinquish the exclusive lock. Must only be called by the thread that called #start_exclusive (and currently holds the lock).
# File lib/active_support/concurrency/share_lock.rb, line 95 def stop_exclusive(compatible: []) synchronize do raise "invalid unlock" if @exclusive_thread != Thread.current @exclusive_depth -= 1 if @exclusive_depth == 0 @exclusive_thread = nil if eligible_waiters?(compatible) yield_shares(compatible: compatible, block_share: true) do wait_for(:stop_exclusive) { @exclusive_thread || eligible_waiters?(compatible) } end end @cv.broadcast end end end
# File lib/active_support/concurrency/share_lock.rb, line 130 def stop_sharing synchronize do if @sharing[Thread.current] > 1 @sharing[Thread.current] -= 1 else @sharing.delete Thread.current @cv.broadcast end end end
Private Instance Methods
Must be called within synchronize
# File lib/active_support/concurrency/share_lock.rb, line 204 def busy_for_exclusive?(purpose) busy_for_sharing?(purpose) || @sharing.size > (@sharing[Thread.current] > 0 ? 1 : 0) end
# File lib/active_support/concurrency/share_lock.rb, line 209 def busy_for_sharing?(purpose) (@exclusive_thread && @exclusive_thread != Thread.current) || @waiting.any? { |t, (_, c)| t != Thread.current && !c.include?(purpose) } end
# File lib/active_support/concurrency/share_lock.rb, line 214 def eligible_waiters?(compatible) @waiting.any? { |t, (p, _)| compatible.include?(p) && @waiting.all? { |t2, (_, c2)| t == t2 || c2.include?(p) } } end
# File lib/active_support/concurrency/share_lock.rb, line 218 def wait_for(method) @sleeping[Thread.current] = method @cv.wait_while { yield } ensure @sleeping.delete Thread.current end